Custom antenna?

I have made myself a door lock, on the outside I have a reader, but today the reader only reads plastic cards.
I have bougth the MFRC-522 / RC522 reader on ebay. this reader works those times I mange to locate the sweet spot with my hand in direct contact with the antenna. however, its impossible to read thougth a thin plastic cap.
My question is now, if remove the current antenna and replace it with my own, how to calculate the custom antenna with the theoretical maximum reading range?
May I need to change some components too?

best regards

These “antennas” are just inductors. They work like a classic LC resonant circuit that resonates at the target frequency (13.56mhz in this case).

So, you need to find out what the capacitance is so you can calculate the inductance you need from your antenna coil… or you can try to read the inductance of the antenna coil you are replacing. Both can be a pain in the ass, depending on circuit design of the reader.

Basically your best geometry for a replacement antenna is a “barrel” style cylinder, which will make it thick and bulky compared to a flat plane spiral coil like what most RFID/NFC antennas are.

I tried to measure them, see picture. I can replace them with other capacistors if that would be better? How big should the coil be in diameter? how many turns? etc. etc.

It’s time to put on an engineering hat… there are many antenna design documents you can reference… the calculations depend on the shape of antenna you want to make;

I’m pretty sure ST even has an antenna design software package that might help.

That was alot more reading then I was hoping for :scream:

I was little bit hoping that someone already had figure this out and maybe could share the design of a better antenna then the integrated one. I can not be the first one that want better reading range with this cheap reader?

cant I just measure lenght of the pcb antenna, and make a round design with wire that has the same lenght? or would that be pointless?

Sorry it’s kinda pointless… the wire diameter and shielding would all play into it. The tough thing about getting an inductance reading is that getting a true inductance would mean buying a very expensive meter. You can find LC meters easy enough, but most do not have sweep frequencies that go much past a few hundred khz… and you’d need to get up to 13.56MHz… and those LC meters are expensive. Still, if you wanted to tinker around a bit, you could try getting an inductance reading using a cheap LC meter, then either calculate things out or just start winding things and measuring as you go until you get as close to the same inductance. The only thing I’d suggest is that you measure inductance of the PCB antenna by destroying it… cut the traces and measure the antenna separate from the rest of the circuit, or you will get some funky results.